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Speedo calibration

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Patrick67BJ8

Patrick67BJ8

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It's time to have my speedo calibrated and I'm going to send it to Nisonger for rebuild and calibration. Peter at Nisonger said to mark off 52' 9 1/2" and count the number of turns the speedo cable turns and do this at least twice to be sure there's no error.

This sounds doable but there's one thing that bothers me...how do you set up to do the measurement to mark off this procedure(stick of wood under the tire, etc), about how many turns of the needle and there's that 1/2" that must be accounted for. This seems like it's really splitting hairs.

I'm going to place a piece of masking tape on the cable needle so that it points upwards to start the process once I get the car in position.
 
Put tape on the ground at both ends and then tape on the wheel so you know its start and finish.
Jerry
 
Patrick67BJ8 said:
.... This seems like it's really splitting hairs.

I'm going to place a piece of masking tape on the cable needle so that it points upwards to start the process once I get the car in position.
Click to expand...
That's because any error is multiplied by 100 times.

You also want to put a piece of paper or cardboard around the cable with 1/10 of a turn on it so you get the number of turns to the nearest tenth.

Finally, do it three times and only while moving forward.
 
Patrick67BJ8 said:
It's time to have my speedo calibrated and I'm going to send it to Nisonger for rebuild and calibration. Peter at Nisonger said to mark off 52' 9 1/2" and count the number of turns the speedo cable turns and do this at least twice to be sure there's no error.

This sounds doable but there's one thing that bothers me...how do you set up to do the measurement to mark off this procedure(stick of wood under the tire, etc), about how many turns of the needle and there's that 1/2" that must be accounted for. This seems like it's really splitting hairs.
Click to expand...

That's just the 5280 feet in one mile divided by 100 and then converting the 0.8 into inches. The closer you get, the more accurately Nisonger can calibrate your speedo. If you really want to split hairs, it's 6/10 inches, not 1/2.
icon7.png
 
I've often wondered why we do it this way. It seems like it would be more accurate to move a set number of turns and then measure the distance you travel for those turns. I'm assuming it would be more accurate because it would be easier to tell when the marker on the cable is exactly straight up (or wherever you started it) than to try and estimate 10ths of a turn. In contrast you can very accurately measure the distance for a set up number of turns.
 
I made this from a piece of stock art online - circle divided into tenths. First I clamp it onto the speedo cable, then loosen the front Allen screw to locate the zero position at the top. Clamping is supplied by bonded rubber washers.
I'm partial to a wire pointer extending from the front bumper close to the pavement. Also use a construction tape measure. One person pushes the car, steers from outside and counts the revs. The other observes the pointer arriving at 52'-9.5". The tape measure is good because it's easy to align for starting. Note since the cable rotates clockwise, the numbers are backwards.
SpeedoCounterF.JPG
SpeedoCounterR.JPG
 
I got the same instructions from Nisonger about the 59 ft thing . However there was 3ft of snow and it was sub zero temps so there was no way I was pushing a car up and down the road .
So I put on the wheels and tires that would be on the finished vehicle and set the pressures . I then marked one tire and rolled the car back and forth in the garage to calculate the distance travelled in one revolution of the wheels . I also noted how many turns the speedo cable tuned in the same distance (I have a 3.54 rear end gear set) .
I then using my limited math skills and a calculator worked out how many wheel turns to travel the 1/10th of a mile etc and sent it all to peter at Nisonger with my calculations . He said perfect and did it for me and its good .
 
I completed my calculations and mailed the speedo to Nisonger.
I had 7 full turns and little bit over 9/10 partial turn. I was expecting a lot more turns but it didn't work out that way.
I have a 3:54 rear axle ratio and 185 x 15 tires. The taller tires add about one inch in height to my BJ8.
 
I'm curious why Nisonger requires this. I had a speedo for a BN2 rebuilt by West Valley and they didn't require this to be done. The shop should have an adjustable speed motor--one of the better Dremel tools can have RPM set--and they can calibrate the speedo on the bench. I 'calibrated' my BJ8's speedo by 1) calibrating the tach to a dwell-tach, 2) using one of many online gear ratio calculators to make a table of RPM<->MPH (in straight fourth), 3) driving the car and moving the indicator until it agreed with my calculated values, more-or-less. I measured my tire diameters at rest and allowed a small fudge factor--about a half-inch--to allow for the diameter increase caused by centrifugal forces. I also check against the roadside radar units when I go by one.

Mechanical speedos, like our Smiths, are usually only accurate in a range (note they don't even move below 10MPH). My BJ8's reads a little high below 40MPH--this might have saved me a speeding ticket or two--then starts to fall behind above 70 (don't drive it that fast very often anyway). I don't know if the 'factory' even calibrated them; if you look closely there is a small white dot below the spring-loaded 'peg,' and I think the assemblers set the indicator (needle) on that dot and pushed the peg in to move the needle above it.
 
After all these years, POs, changes to tire size and rear end ratios, they can't be sure what the ratios are. My speedo cable turns about 13 turns in 1/100 of a mile.
 
Bob_Spidell said:
I'm curious why Nisonger requires this. I had a speedo for a BN2 rebuilt by West Valley and they didn't require this to be done....
Click to expand...

Bob - When I moved from 185/70s to 180/85s (taller), I had mine done recently at West Valley and Morris needed the number of turns. Were your tires the stock diameter and that's why it was not necessary?
 
Hi Steve,

I don't recall Morris asking any questions when I sent our BN2 to him for calibration. Klaus at Palo Alto Speedo didn't ask when I had new gears installed for the 3.54 rearend I installed any years ago.
 
Bob_Spidell said:
Hi Steve,

I don't recall Morris asking any questions when I sent our BN2 to him for calibration. Klaus at Palo Alto Speedo didn't ask when I had new gears installed for the 3.54 rearend I installed any years ago.
Click to expand...

Hi Bob,
I've had mine done by MoMa once, Nisonger at least twice and West Valley once - for 2 Toyota transmissions, 3.54 diff and tire size changes.
Morris explained the speedo is only a rev counter and anything that changes the number of revs per mile necessitates re-calibrating.

If one left the speedo internal gears the same when switching from a 4.11 to a 3.54, the indicated speed would be 86% of that previously shown given the same size tires.
 
steveg said:
... Morris explained the speedo is only a rev counter and anything that changes the number of revs per mile necessitates re-calibrating.

If one left the speedo internal gears the same when switching from a 4.11 to a 3.54, the indicated speed would be 86% of that previously shown given the same size tires.
Click to expand...

Yep. The internals of the tachs and speedos are almost the same; the only difference is the speedos have geared ratchet mechanisms to drive the odometers and trip meter. The only adjustment that I'm aware of is placement of the indicator. There is one component in the speedos--tachs may have it too, it's been a while since I've gotten into one--that I don't understand: the only thing that holds the tiny shaft that turns the indicator is a parallel, brass bracket (crappy ASCII art follows):

___X_|_____
\__X_|____/


(the misaligned vertical bars are supposed to represent the indicator drive 'shaft'). The Xes represent a tiny screw that appears to be used to adjust the two parallel parts of the bracket (represented by underscores in my crappy artwork). The only thing I can figure is the screw is used to 'pinch' the two parts of the bracket, which should change the angle of the indicator drive cup to the rotating magnet, which might compensate for variances in different speed ranges (even the 'famous' documentation of the Smiths speedos doesn't mention it).

"If one left the speedo internal gears the same when switching from a 4.11 to a 3.54, the indicated speed would be 86% of that previously shown given the same size tires."

Yep; except I went from the stock 3.91 in a BJ8 to a 3.54, which I calculated was an 11% difference. When Klaus changed the internal gears for me he neglected to recalibrate the indicator, and I passed a California Highway Patrolman because I thought he was doing 50 in a 55MPH zone, and he promptly pulled me over. I was so ******-off at the speedo I handed the CHiP my American Express card instead of my driver's license; and when I explained the problem he laughed and sent us on our way.

Note your speedo--carefully calibrated to your exact tire diameter--will become increasing incorrect as your tires wear down, or you install new ones with a different diameter. First thing I do when I take my long road trip every year is to 'calibrate' my speedo against my co-pilot's GPS.
 
Does the type of transmission come into play when getting a speedo calibrated? I also went to a Toyota 5 speed but 4th gear is driect just like the original Healey trans.
 
Am I wrong to assume that the car is in top gear while making this 52+ roll? I'll be recalibrating when I get my new 3.54 differential installed on a '60 BN7 with a 50-speed Toyota box. I managed to blunder the speedo almost spot on with the present differential when I reinstalled the speedo needle after acquiring the car. It was reading at least 10 mph off prior to that good fortune. Just good luck on my part. I use my GPS to establish my actual speed vs. the indicated speed.
 
The only thing that matters is the driveshaft final output rotational speed. However, if you want to calibrate tach reading (RPM) versus speedo reading using the ratio calculators, you need to know the transmission gear ratio, final drive ratio and tire diameter.
 
Bob_Spidell said:
The only thing that matters is the driveshaft final output rotational speed. However, if you want to calibrate tach reading (RPM) versus speedo reading using the ratio calculators, you need to know the transmission gear ratio, final drive ratio and tire diameter.
Click to expand...
This info is for mechanical speedos?
 
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